1. Field of the Invention
The present invention relates to a vehicular constant-speed running system for changing the speed control of a vehicle from manual control by the driver to automatic control, e.g., "cruise control" (hereinafter "C/C").
2. Description of Related Art
Japanese Patent Publication No. Hei 3-16292 discloses a vehicular constant-speed running system in which the opening of the throttle valve (hereinafter "throttle opening") is initially set at the change of the vehicle speed control from manual to automatic. A tentative target opening of the throttle opening at the C/C set time is stored in advance based on a target vehicle speed so that it is used for the vehicle speed control at the initial stage of the C/C setting.
In this system, the tentative target opening, as stored in advance, is a throttle opening corresponding to the target vehicle speed at the time when the vehicle is running on a flat road. If the C/C setting is made during an uphill or downhill run, the initial value of the throttle opening will deviate significantly from the proper value. As a result, it takes a long time for the actual vehicle speed to coincide with the target vehicle speed using vehicle speed feedback. This is undesirable for the driver.
It is possible that the throttle opening immediately before setting is used as the initial value of the throttle opening at the C/C setting time. According to this method, it is possible to avoid the above-described problem occurring during an uphill or downhill run. Assume that the driver depresses the accelerator pedal after the run at a steady speed by manual control to perform the C/C setting while the vehicle is accelerating. At this time, the initial value of the throttle opening to be used for C/C is far larger than that necessary for the steady run at the vehicle speed for the C/C setting. As a result, it takes a long time for the actual vehicle speed to coincide with the target speed using vehicle speed feedback. This also may make the driver uncomfortable.
Here, according to the general output torque characteristics of the internal combustion engine, the relationship between the throttle opening and the output torque is shown in FIG. 22. Depending upon the engine rotational speed NE, the output torque is saturated for the smaller throttle opening especially in the lower ranges (e.g., where NE=N1). This makes the C/C feedback gain match the region in which the torque gradient before the saturation of the output torque changes substantially proportionally (with reference to the gain compatibility point of FIG. 22). In other words, it is assumed that by opening and closing the throttle opening to some extent that the output torque of the internal combustion engine increases or decreases correspondingly to respectively accelerate or decelerate the vehicle. However, once the throttle opening is initialized to such a large value that the output torque is saturated, the decrease in the output torque delays even if the throttle opening is driven toward the closed direction by the preset feedback gain. As a result, it takes a long time for the actual vehicle speed to coincide with the target speed.